“Electric power assisted wheelchair”which assists the driving force by electric motors is expected to be widely used as a mobility support system for elderly people and disabled people. This paper aims at realizing the safe and reliable step climbing motion control. The reason is that power assisted wheelchairs are in danger of backward overturning when raising the front wheels due to the excessive driving torque and of stopping in front of steps when the rear wheels try to climb over steps due to the insufficient driving torque. This study proposes a novel step climbing control scheme for electric power assisted wheelchairs based on driving mode switching. The first driving mode,“front wheel raising mode”, applies torque control system with the variable assistance ratio to prevent the dangerous overturning. The second mode,“rear wheel climbing mode”, applies position control system based on the polynomial trajectory to surely climb over steps and stop at the suitable position. Some experiments show the effectiveness of the proposed control system.